The present invention relates to a system for extracting oil, comprising a well for extracting oil from an oil reservoir, a separator device for separating oil and water out of an extracted mixture that comprises water and oil, said separator device comprising a deviated first flow path for said mixture and being arranged along a deviated portion of the well, and a second flow path for receiving water or a water enriched phase that has been separated from the oil-water mixture in the separator device. The separator device comprises a plurality of draining openings along a section of the deviated first flow path, via which the water or water enriched phase flows under the action of gravitational forces from the first flow path to the second flow path.
The invention also comprises a method of extracting oil from an oil reservoir, comprising the steps of extracting a liquid mixture comprising oil and water from the reservoir via a first flow path in a well, and separating, under gravity in a deviated section of the well, the liquid into separate streams one of which mainly comprises water or a water enriched phase, the water or water enriched phase being passed from the first flow path to a separate second flow path via a plurality of draining openings along a section of the deviated first flow path.
The system and method particularly relates to down-hole separation of an extracted oil-water mixture in any oil field, on land as well as off-shore.
The term oil-water mixture should be regarded in a broad sense, and it should be understood that such a mixture is also likely to contain gaseous components such as natural gas as well as solids such as sand particles. The water or water enriched phase separated from the mixture may also contain such further components.
The majority of oil reservoirs world-wide start to produce water as they mature. The water/oil ratio, i.e. the water cut, varies with geographical location and the nature and age of the reservoir. As the number of mature fields increase, the industry is facing a need for techniques that ensure economical and efficient production of oil with increasing water cut.
The lifecycle cost for an oilfield can be significantly reduced if the available topside process plant is dedicated to oil production all through the life of the field. If the water cut in the incoming stream is reduced, this may create capacity that will allow tie-in of additional wells, or increased production from existing wells.
The fact that reduced water cut enables increased production of oil is the essence of the drive towards separation of oil and water prior to the entering of the wellstream on the topside facilities.
Down hole separation will in many cases enhance the oil production because, for example, the tubing head pressure will increase significantly as the water is removed down hole, and the increased tubing head pressure will be used to increase the flow of oil from the well. Alternatively the pressure of a first stage gravity separator which might be included in the system, for instance arranged on a topside installation, will be increased, and thus the gas flashed off in the first stage separator will need less compression before being injected or exported.
Gravity separation is in many ways an advantageous solution to separation in the well since this is an extension of the natural separation in the wellbore.
A method and apparatus of separating the components of the fluid produced by an oil well which comprises down hole separation under gravity in a deviated non-vertical section of a wellbore is disclosed in GB 2 326 895, to Schlumberger Limited. According to this document, at least two separate flow paths having openings to the flow of the fluid at an upper end of or within a non-vertical section of the well are provided. The gravity is allowed to separate the fluid flow into a hydrocarbon enriched part and a water enriched part. The hydrocarbon-enriched part is flowing through the upper of the vertically separated openings, and the water-enriched part is flowing through the lower of the separated openings.
However, according to the above document, all the water or water enriched part is separated from the oil enriched part at one single location. For practical flow rates, a large amount of the hydrocarbon-enriched part, mainly oil, will follow the water-enriched part, and hence there will be an undesired reduction of the separation efficiency. When, for example the water enriched part is re-injected into an oil reservoir from which the fluid is extracted, this means that also oil that has already been extracted is re-injected into the reservoir, which is an undesirable effect for obvious efficiency reasons.
It is an object of the invention to provide a system for extracting oil which comprises a gravity separator device for down hole separation of water and oil that shall promote an efficient separation of water from oil by means of gravitational separation at an early stage after that an oil-water mixture has been extracted from an oil reservoir, and that is robust and represents an advantageous alternative to prior art separators from an economical point of view.
The object of the invention is achieved by means of the initially defined system, which is characterised in that the draining opening area per area unit decreases in the flow direction of the oil-water mixture along said section of the deviated first flow path.
Thereby there will be a pressure compensation between the draining openings, which will promote a large separation capacity of the separator. It should be understood that the draining openings are distributed in the flow direction of the oil-water mixture and at different altitude levels. In this context, openings are referred to as slots in a wall arranged between the first and second flow path, but may have other implementations such as holes or perforations. Preferably such a wall is the wall of a tube or tubing that encloses and defines the first flow path.
The draining opening area per area unit decreases in the flow direction of the oil-water mixture along said section of the deviated first flow path. If, for example, the draining openings comprise slots or holes in a wall section between the first and second flow path, the distance between such openings may be increased and/or the individual size of such openings be decreased in the flow direction of the oil-water mixture in order to accomplish this feature. Thereby, less oil will follow the water or water enriched phase through the draining openings to the second flow path than would otherwise be the case, as consideration is taken to the changing separation conditions that exist along the draining section due to changing pressure conditions and concentration changes in the oil-water mixture that passes through that section.
According to a developed embodiment, the draining openings are distributed along a distance of at least 100 times the length of the diameter of the first flow path. In general terms, the basic idea is to provide a draining section long enough to ensure that the water in the oil-water mixture gets time to separate due to the gravitational forces, and to form a water or water enriched layer in a lower part of the first flow path. Then, by means of the draining openings, the water is continuously drained off from the first flow path along the draining section. When optimising the configuration and distribution of the draining openings consideration is taken to the flow rate of the oil-water mixture in the first flow path.
According to one embodiment the system of the invention is characterised in that, at least in the section along which the draining openings are located, the cross section of the first flow path is locally expanded such that, under the prevailing pressure conditions in the well, a locally reduced flow rate of the oil-water mixture is obtained along said section. Flow rate is referred to as flow velocity measured in m/s and should be low enough to permit a gravitational separation of water along the draining section. A preferred flow rate, in order to achieve a successful separation and draining, is below 3 m/s, preferably below 1 m/s.
Preferably the first flow path is defined by a first tube, and the system comprises a second tube which encloses the first tube and defines an annular path between itself and the first tube, wherein the annular path comprises the second flow path. Preferably, a conventional production casing that surrounds the production tube or tubing forms the second tube. Such a solution is advantageous both from an economical and technical point of view. The system may also comprise further tubing, at least partly arranged in the annular path between the first and second tube, for further transportation of the water or water enriched part separated from the oil-water mixture, thereby defining a continuation of the second flow path.
According to a further embodiment, the second flow path comprises a path for re-injection of water to the oil reservoir. The water re-injected is the water that has been separated from the oil-water mixture in accordance with the teachings of the invention. Thereby advantages already discussed in the introductory part of this application are obtained. Depending on the prevailing conditions in the reservoir, the path for re-injection is arranged so as to transport the water back into the reservoir via the same well as the oil-water mixture has been extracted through or, alternatively, via a different well branch or a different well so as to transport the water back to the reservoir at a given distance from the well via which the oil-water mixture has been extracted. The distance should be long enough to ensure that the re-injected water is not immediately re-circulated into the well. As a further alternative, the path for re-injection may be substituted by a path for discharge of the water into the sea.
The invention also relates to a method of extracting oil. The inventive method shall promote an efficient separation of water from oil by means of gravitational separation at an early stage after that an oil-water mixture has been extracted from an oil reservoir. It is also an object of the invention that the method shall result in a minimum of oil being separated and drained off together with the water from the oil-water mixture. The method shall permit an oil-water mixture flow rate that is acceptable from a practical and economical point of view without having an unacceptable amount of oil drained off together with the water.
This objective is achieved by means of the initially defined method, which is characterised in that it comprises the step of providing a decreasing draining opening area per area unit in the flow direction of the oil-water mixture along said section of the deviated first flow path.
The water or water enriched phase shall be drained off from the oil-water mixture at different altitude levels along the deviated first flow path in order to make it possible to continuously drain off water that, due to for instance the mixture configuration, settles with different rates in the gravity separator formed by the deviated first flow path. Preferably, the water is drained off via openings that are distributed in the flow direction of the oil-water mixture in the first flow path.
Further advantages and features of the present invention will be described in the following detailed description and in the appended claims.